1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a method and apparatus for call admission control to guarantee communication quality in a cellular communication network such as a CDMA (Code Division Multiple Access) communication network.
2. Description of the Related Art
CDMA is a multiple access technology most widely used in post-2.5 generation cellular networks. From the system implementation point of view, a CDMA communication network is characterized by soft capacity, as compared to TDMA (Time Division Multiple Access) and FDMA (Frequency Division Multiple Access). In the CDMA network, each user is identified by a unique code and if a set of codes with orthogonality is used, as many users as the available codes can theoretically be serviced concurrently. In a forward link (or an uplink), however, it is impossible to use orthogonal codes due to different propagation delays caused by different distances between users and a base station. Thus, other users' signals are noise to another user.
In digital communication, signal-to-noise ration (SNR) determines bit error rate (BER). As more users are serviced simultaneously, the BER for each user is higher. In real implementation, data is delivered on a frame basis. If a BER is increased, it leads to an increase of a frame error rate (FER). Therefore, the FER must be maintained at or below a predetermined value in order to provide acceptable communication quality to the user. On the other hand, this means limitation of the number of users to be simultaneously serviced. That is, the maximum number of users to be serviced simultaneously is not fixed but depends on the communication quality that the system guarantees. This system characteristic is termed soft capacity.
In view of the soft capacity, the CDMA communication system has a variable capacity under the circumstances. The circumstances relate to noise. For example, noise factors include intra-cell interference, inter-cell interference, and background noise containing thermal noise. Among the noise factors, it is impossible to accurately estimate the inter-cell interference. In this context, a call setup request must be admitted or rejected adaptively according to existing conditions in order to maintain FER at or below an acceptable level. This process is usually called “call admission control (CAC)”.
A traditional CAC operation is disclosed in Klein S. Gilliousen, “On the Capacity of a Cellular CDMA System,” IEEE Transactions on Vehicular Technology, vol. 40, pp. 303–312, May 1991 (reference 1), which is incorporated herein by reference. Reference 1 analyzes CDMA system capacity and presents the maximum number of users that can be accommodated based on a given BER being satisfied. While reference 1 does not deal with CAC explicitly, it is substantially about CAC in that, as illustrated in FIG. 1, which is a flowchart illustrating an example of steps for performing a conventional CAC operation, the maximum number of users to be serviced concurrently is decided. Only call setup requests from users equal to or less than the maximum number are admitted. According to reference 1, the maximum user number is fixed irrespective of network conditions. Hence, a CDMA system adopting the CAC technique is equivalent in effect to a TDMA/FDMA system.
Z. Liu and M. E. Zarki, “SIR-based Call Admission Control for DS-CDMA Cellular Systems,” IEEE Journal on Selected Areas in Communications, vol. 12, pp. 638–644, May 1994 (reference 2), which is incorporated herein by reference, provides another traditional CAC operation. According to reference 2, upon receipt of a call setup request, SNR is measured. Only if the SNR is great enough to satisfy a required BER, will the call setup request be admitted as illustrated in FIG. 4 which is a flowchart illustrating an example of steps for performing another conventional CAC operation.
Reference 1 and reference 2 commonly aim to maintain BER at or below a desired value. While it is decided indirectly whether a call setup request can be admitted according to the number of users closely related to noise in reference 1, the decision is directly made according to SNR being a quantitative measurement of BER in reference 2.
A cellular mobile communication system such as CDMA primarily seeks to maintain FER at or below a particular value. Yet, this is not the only purpose of CAC. Although a user wants to receive seamless service, e.g., crossing a cell boundary from one cell to another cell, if the new cell is already full of users, the shortage of resources may bring about service interruption for the user. The cell boundary crossing during a call is called handoff. Needless to say, the number of handoff call drops decreases user satisfaction markedly. Hence, CAC must be performed such that the handoff call dropping probability is diminished, for example, by reserving part of network resources for a handoff call.
As described above, CAC aims to decrease both FER and handoff call dropping probability at or below predetermined values. Hereinafter, an FER and a handoff call drop rate at or below the predetermined values are referred to as “frame-level quality” and “call-level quality,” respectively. Despite the equal significance of frame-level quality and call-level quality, most traditional CAC operations consider only one quality. Some CAC operations, even if they consider the two qualities, fail to actually ensure them. Particularly, reference 1 and reference 2 exhibit the following shortcomings.
Reference 1 aims to satisfy the frame-level quality and presents the call admission condition that the current number of users must be less than a preset maximum user number. BER is determined by SNR, as described before. On the assumption that signal strength is constant by power control, noise determines the SNR. Among the noise factors of intra-cell interference, inter-cell interference, and background noise, only the intra-cell interference can be estimated from the current user number. Usually, the background noise can be estimated but the inter-cell interference varies depending on circumstances. For example, network conditions such as load and the spatial distribution of users affect the inter-cell interference. The network load generally varies with time and geographical areas. Moreover, the user spatial distribution is not uniform due to the influence of topography. It is not easy to preset the maximum number of users that can be accommodated by taking all these considerations into account. Consequently, handoff call dropping probability, which is a criterion of deciding call-level quality, is neglected in reference 1.
FIG. 2 is a graph illustrating FERs in the case of a uniform user spatial distribution in a simulation of the conventional CAC operation illustrated in FIG. 1. Specifically, FIG. 2 illustrates FERs versus load in the case of a uniform user spatial distribution with a maximum user number of 38, 40, and 42 in a simulated CAC according to reference 1. Referring to FIG. 2, as network load increases, FER also increases. Therefore, unless a maximum load on a cell can be estimated, the maximum user number cannot be decided.
FIG. 3 is a graph illustrating FERs in the case of a non-uniform user spatial distribution in the simulation of the conventional CAC operation illustrated in FIG. 1. Specifically, FIG. 3 illustrates FERs versus load in the case of a non-uniform user spatial distribution in the simulated CAC according to reference 1. Referring to FIG. 3, “non-uniform” indicates a concentration of users 75% outside of a cell. “Hot cell” indicates a cell having a load greater than twice the other cells. As noted from FIG. 3, when users concentrate outside the cell, FER rapidly increases with load. Therefore, if the spatial distribution of users cannot be predicted beforehand, the maximum number of users cannot be decided.
Reference 2 also aims to guarantee frame-level quality and presents the call admission condition that SNR must be higher than a threshold at the moment when a call admission request is received. Reference 2 advantageously reflects various network conditions because noise is directly measured. Because the SNR is a random process that changes rapidly, making the decision about call admission by taking into account only the sample value at the moment of receiving the call setup request may lead to errors. Reference 2 also neglects the handoff call dropping probability.
FIG. 5 is a graph illustrating FERs versus target FERs in a simulation of the conventional CAC operation illustrated in FIG. 4. Specifically, FIG. 5 illustrates FER measurements versus target FERs in a simulated CAC operation according to reference 2. Referring to FIG. 5, with a small network load, an FER is lower than a target FER, but with a large network load, the FER exceeds the target FER.
FIG. 6 is a graph illustrating FERs in the case of a non-uniform user spatial distribution in the simulation of the conventional CAC operation illustrated in FIG. 4. Specifically, FIG. 6 illustrates FERs versus loads in the case of a non-uniform user spatial distribution in the simulated CAC operation according to reference 2. Here, a target FER is set to 0.01. Referring to FIG. 6, while the influence of user concentration outside the cell is not so great as compared to the CAC according to reference 1, as network load increases, an FER measurement exceeds the target FER, sensitively responding to the network load.